Along with the miniaturization of portable apparatuses such as portable telephones and PHS, there are ever-increasing demands for light weight and compactness of light emitting devices and other devices used therein, and small-size thin chip-type light emitting devices have been developed.
As illustrated in FIG. 5(a), in the small-size, thin, and chip-type light emitting device, terminal electrodes 1 and 2 are formed on both ends of a substrate 10, a light emitting chip (hereinafter, referred to as LED chip) 3 is die-bonded by silver paste, transparent epoxy resin, etc. onto an electrode electrically connected with one of the terminal electrodes 1 so as to form one part of the terminal electrode, with its lower electrode being directly connected to the terminal electrode 1 and its upper electrode being wire-bonded to the other terminal electrode 2 by a gold wire 4; thus, the respective parts are electrically connected. An insulating substrate, made of a material such as, for example, a BT resin material in which a glass cloth is impregnated with a heat-resistant BT resin, is used as the substrate 10. Moreover, for example, as illustrated in FIG. 5(b), a pn junction face (light emitting layer) 43 is formed by joining an n-type semiconductor layer 41 made of GaAs, Gap, etc. to a p-type semiconductor layer 42, and electrodes 44 and 45 are formed on the respective surfaces thereof; thus, the LED chip 3 is formed. A package 6, which covers and protects the LED chip 3 and the gold wire 4, is formed on the surface of the substrate 10 by a resin such as a transparent or milky-white epoxy resin.
Along with the development of light, thin, and small-size electronic apparatuses as described earlier, there have been strong demands for further miniaturized chip-type light emitting devices and for LED chips made as small as possible. Further, with respect to those devices, high performance and high luminosity have also been demanded. Here, the conventional chip-type light emitting device is manufactured by mounting and wire-bonding it on the surface of one of the terminal electrodes as described above. Since these terminal electrodes are formed by applying gold plating onto copper patterns, they tend to absorb light that has reached the back side of the substrate of the LED chip 3. In particular, in the case of bluish-colored LED chips which use sapphire substrates, light reaches the back side without being attenuated so much, and the light that has reached the back side tends to be absorbed since bluish lights are easily absorbed, in particular, by yellowish (gold) colors. The resulting problem is that light emitted by bluish-colored LED chips is not utilized efficiently.
As illustrated in FIG. 6(a) that is a partial cross-sectional front view, a lamp-type light emitting device is constructed as follows: A pair of leads 11 and 12, which are made of an iron material to which silver plating is applied, are installed, a mount section 15 is formed on the top portion of one of the leads 12, and a recessed section 15b is formed virtually in the center of the mount section 15. A LED chip 3 is die-bonded to the inside of the recessed section 15b by using a bonding material such as silver paste or transparent epoxy resin. With respect to the LED chip 3, a chip having a red, green or other color, which is provided with upper and lower electrodes, is used. Here, in FIG. 6(a), a bluish-colored LED chip, which is constructed by stacking layers made from a gallium nitride based compound semiconductor such as GaN on a sapphire substrate, is exemplified. Therefore, two electrodes are installed on the upper surface side, and electrically connected to the pair of leads 11 and 12 respectively by metal wires 4 such as gold wires. Further, the peripheral portion is molded by a transparent or translucent (transparent to light-emitting wavelengths of the LED chip) synthetic resin so that a package 18 made of the resin is formed. This package 18 is provided with a lens section 18a having a virtually hemispherical shape on its top portion.
The back surface of the LED chip 3 is bonded by a bonding material 20 such as silver paste or transparent epoxy resin, and light proceeding toward the back surface side is reflected as shown in FIG. 6(b). Light, emitted from the LED chip 3, mostly proceeds straight forward from the top surface, is refracted by the lens section 18a of the package 18, and further emitted outward while being converged in the direction of the center axis. One portion of the light emitted from the LED chip 3 proceeds toward the back surface of the LED chip 3 (refer to E), and is reflected by the bonding material 20 or the bottom surface of the recessed section 15b, thereby forming reflected light R. The reflected light R also proceeds forward from the top surface of the LED chip 3, is refracted by the lens section 18a and emitted outward.
The conventional lamp-type light emitting device has such a structure, and in the case when silver paste is used as the bonding material 20, the light E, which has proceeded toward the back surface side, is reflected by the silver paste. In the case when a transparent epoxy resin is used as the bonding material 20, it passes through the bonding material 20, and is reflected by the silver plating 15a of the bottom surface of the recessed section 15b. However, the reflection rate of the silver paste or silver plating is not so great. For this reason, the light that has proceeded toward the bottom surface side of the LED chip 3 is not utilized efficiently, resulting in a problem in which the efficiency of light to be emitted outward is not enhanced sufficiently.
Moreover, the thermal conductivity .lambda. (cal/(m.h.deg)) of silver is in the range of 300 to 400, while that of epoxy resin is 0.27. Therefore, when a LED chip is die-bonded to a fitting member by a transparent epoxy resin, heat radiation from the light emitting device is not carried out favorably, occasionally resulting in discoloration of the transparent epoxy resin. Such discoloration of the transparent epoxy resin also causes an increase in the loss of light that is emitted from the LED chip.